Arrangement for supplying current to a subscriber telephone set

ABSTRACT

An apparatus for supplying current to a subscriber telephone set (R L ) connected to a telephone exchange via a two-wire line departing from a a-wire and a b-wire output terminal in the arrangement. The arrangement includes two analogue amplifiers (2, 3) each with its output connected to said respective a-wire and b-wire output terminal, through which the combined DC-current and current representing the speech signal are fed out to the line. The apparatus further includes a voltage source with the pole voltage (E B ) and its poles connected to two voltage supply inputs on each amplifier (2, 3), the output of the first amplifier (2, 3) being kept at a fixed potential. Three signal sources (5, 6, 8) are connected to the inputs of the amplifiers, the respective signal contribution of these sources being added for controlling the output magnitudes of the amplifiers. A first signal source (6) constitutes the reference signal source and has a fixed signal contribution. A second signal source (5) is controlled by the output magnitudes of the amplifiers, and a third signal source (8) is controlled by the difference between the transversal line voltage and the actual voltage (E B ).

TECHNICAL FIELD

The present invention relates to an apparatus in the current supply of asubscriber's telephone set from a telephone exchange.

BACKGROUND ART

In telephone installations the subscriber sets are connected to theexchange by so-called subscriber lines, which are terminated in theexchange at an adaption circuit, or subscriber line interface circuit(SLIC). Such a circuit is used, inter alia, to supply DC current to theassociated subscriber apparatus.

A fixed DC voltage may be utilized for this current supply in the priorart. In such a case, the current flowing through the subscriber linewill be a function of the internal resistance of the subscriberapparatus or station, and of course the line length. The internalresistance in the apparatus can be kept within narrow limits, but theline length and line resistance may vary considerably. If a fixed DCvoltage is used, this must be given a value such that sufficient currentcan be fed out on long lines as well.

In modern line circuits, the supply impedances are often simulated withthe aid of feed-back amplifiers. The physical supply resistances arelow-ohmic and the power generation in the resistance is low. In order toreduce the power generation in the output step of the amplifier as well,it is known to utilize controllable DC/DC converters, e.g. so-calledchoppers, instead of batteries in the voltage supply.

Two principles are described in the literature for the impedancesimulation, c.f. Proceedings Of the IEEE, Vol. 68, No. 8, August 1980,pages 991-1009, for example. According to one principle, the linecurrent is sensed to form an instant value in a control systemcontrolling the line voltage. This method gives simple conditions foroperating point setting of the participating amplifiers and control ofDC/DC converters. However, the method has the considerable disadvantagethat the amplifier which has floatingly to sense the line current out onthe line will be difficult to achieve with sufficient precision.

According to the other method, the line voltage is sensed and thecontrol system controls the line current. This method gives more simpleconditions for realizing the sensing amplifier, but on the other hand itgives difficulties in setting the operating points of the amplifiers andcontrol of DC/DC converters.

In electronic subscriber line circuits according to the above, theamplifiers require a supply voltage for their function which exceeds bya given amount the transversal line voltage, i.e. the voltage betweenthe outputs of the amplifiers. If this condition is not met, there isthe risk of the signal voltage being clipped. This relationship definesthe maximum length of a line which can be connected to the line circuitwithout risk of transmission degeneration. Consideration must also betaken here of the fact that the battery voltage can vary considerably,depending on the state of charge, load etc.

DISCLOSURE OF INVENTION

In the present invention, the line-feeding two-pole is implemented withthe aid of two analogue amplifiers, through which the combined DCcurrent and current representing the speech signal are fed out on to theline. The DC line feed voltage, as with the internal voltage supply tosuch amplifier, is taken from a chopper or a battery. The inventiveprinciple is, however, also applicable to systems utilizing two choppersper line. It is also applicable to the arrangement where the meanpotential at one output terminal of the subscriber line circuit is keptat a constant level by a control system, not illustrated here.

The technical problem which is solved by the present invention residesin preventing the amplifiers from becoming saturated and thus clippingthe signal voltage, even if a very long line were to be connected to theline circuit and the battery voltage vary.

An advantage of the invention is that the available battery voltage isoptimally utilized at every instant. The telephone system thus functionsin a desired manner for an optimum line length interval and for linelengths exceeding this interval the system still functions, although notas well from the quality aspect.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in detail with reference to theaccompanying drawing, whereon

FIG. 1 illustrates an electronic subscriber line circuit with voltagesensing and current supply,

FIG. 2 illustrates the current-voltage characteristic for the circuitaccording to FIG. 1,

FIG. 3 illustrates an embodiment of the invention,

FIG. 4 illustrates the current-voltage characteristic for the circuitaccording to FIG. 3,

FIG. 5 illustrates an electronic subscriber line circuit with currentsensing and voltage supply,

FIG. 6 illustrates the current-voltage characteristic for the circuitaccording to FIG. 5,

FIG. 7 illustrates an embodiment of the invention,

FIG. 8 illustrates the current-voltage characteristic for the circuitaccording to FIG. 7.

BEST MODES FOR CARRYING OUT THE INVENTION

The principle of the invention will be illustrated below, as applied toa subscriber line circuit, for which the voltage supply to the linecircuit is represented by the symbol for a fixed DC voltage. In thisconnection, however, the symbol must also stand for system solutionswith one or two DC/DC converters connected in series with the source ofsupply.

FIG. 1 illustrates an electronic subscriber line circuit with voltagesensing and current supply. A condition for the continued discussionhere is that that the point A in the figure, i.e. the a-terminal of thesubscriber line circuit, is given a fixed means potential with the aidof a control system CS. This system can be implemented as will beapparent from the Swedish patent application SE No. 82.04185-6corresponding to U.S. patent application Ser. No. 06/598,322, filed Mar.6, 1984.

Via its a and b terminals, the subscriber line circuit supplies a load,e.g. in the form of a telephone set with line, symbolized here by aresistor with resistance .R_(L). Each terminal is connected to theoutput of an analogue amplifier 2,3, which is coupled as a currentsource with the current gain factor A_(i). Via its final stage, each ofthese amplifiers can drive positive or negative current via on the lineto the load from an earth-connected voltage terminal on the amplifier,and a voltage terminal connected to the minus pole on a plus-earthedvoltage source 7 with the voltage E_(B). The output current from theamplifiers 2,3 is controlled via their inputs, to which two currentgenerators 5,6 are connected via a lowpass filter 4. The currentgenerator 6 is a fixed reference current source, supplying the currentI_(o), while the generator 5 is a voltage-controlled current source withthe transconductance G_(t). A differential amplifier 1 is arranged tosense the transversal line voltage U₁, and via its output to control thecurrent generator 5. The following is applicable for the line current I₁:

    I.sub.1 =A.sub.i (I.sub.o -U.sub.1 ·G.sub.t).

FIG. 2 illustrates I₁ as a function of U₁. It will be seen that thesubscriber line circuit simulates a resistive supply with a batteryvoltage equal to I_(o) /G_(t) and a supply resistance equal to 1/A_(i)G_(t). When the transconductance G_(t) is equal to zero there isobtained constant current supply and a horizontal characteristic. If thevoltage E_(B) were to drop, for reasons described above, there may beproblems concerned with clipping the signal voltage for long lines, i.e.for low line current I₁.

FIG. 3 illustrates an embodiment of the invention: A non-linearcomparator 8 with a current output is arranged parallel to the currentsources 5,6 and drives current in the same direction as the source 5.The comparator has two inputs. One input is supplied the voltage -U₁ andthe other a voltage -E_(B) +U_(lim) is an internal voltage reference inthe comparator, here depicted as an external voltage source. Thecomparator is such that for

    -U.sub.1 <-E.sub.B +U.sub.lim

its output current will be

    I=-G.sub.lim (U.sub.1 -E.sub.B +U.sub.lim)

where G_(lim) is the transconductance for the comparator. The outputcurrent is equal to zero for remaining values of the voltage U₁.

Analogously with the preceding there thus obtained for

    U.sub.1 >E.sub.B -U.sub.lim

    I.sub.1 =A.sub.i (I.sub.o -U.sub.1 ·G.sub.t -(U.sub.1 -E.sub.B +U.sub.lim)G.sub.lim)

This function is illustrated graphically in FIG. 4. The characteristichas a break point which is at a distance U_(lim) from the batteryvoltage E_(B), thus following the variations of the battery voltage. Thecontrol system, which keeps the potential on the a-terminal constant, isdimensioned such that this potential will be equal to -U_(lim) /2, andfurthermore, if the voltage U_(lim) is put equal to the voltage withwhich the actual battery voltage must exceed the actual transversevoltage supplemented by the slope of the characteristic for ensuringthat the amplifiers do not become saturated, the above-mentioned problemis avoided.

The inventive principle, which has been illustrated here on anunsymmetric system, may also be realized in a system with a symmetricsupply.

An electronic subscriber line circuit with current sensing and voltagesupply is illustrated in FIG. 5. In the dual method, the output currentis thus sensed from the circuit by a combination of two transconductanceamplifiers 1 and an adding circuit 10. The transfer impedances for theamplifiers 1 is equal to R_(t). In this case the analogue amplifiers aretwo voltage amplifiers with the gain factor A_(v). These amplifiers aresupplied on the input side by two series-connected voltage sources, afixed reference voltage source with the voltage U_(o) and a voltagesource controlled by the output current from the circuit. The outputvoltage from the voltage source 5 is equal to I₁ ·R_(t). The followingis applicable for these conditions: ##EQU1## FIG. 6 illustrates thisrelationship graphically.

An implementation of the inventive idea for the dual system according toFIG. 5 is illustrated in FIG. 7. In this case the comparator 8 has avoltage output and adds a voltage to the inputs of the amplifiers 2,3.When U₁ >E_(B) -U_(lim) with the same denotations as previously, thecomparator output voltage U_(k) is

    U.sub.k =k(U.sub.1 E.sub.B +U.sub.lim)

otherwise U_(k) =0

U₁ may then be written

    U.sub.1 =A.sub.v (U.sub.o -I.sub.1 -R.sub.t -k(U.sub.1 -E.sub.B +U.sub.lim))

This relationship is illustrated graphically in FIG. 8. In this casealso, the characteristic has a deflection point, the position of whichfollows the instant battery voltage E_(b) at the distance U_(lim), andthis relationship gives the advantages mentioned above.

By putting a lowpass filter 4 between the inputs of the amplifiers 2,3and the outputs from the current- and voltage sources, respectively,which control the output magnitude from these amplifiers, the linecircuit from the terminal side is made independent, seen from the signalaspect, of the characteristic being given a deflection point inaccordance with the inventive idea.

We claim:
 1. A telephone exchange system apparatus for supplying currentto a subscriber telephone set connected via a two-wire line to first andsecond terminals of the telephone exchange, said apparatus comprising:first and second analog current amplifier means each having a firstinput, each also having an output connected to one of said terminals foremitting signals, and each further having voltage supply input means;voltage supply means with an output voltage E_(B) connected to each ofsaid voltage supply input means; means for maintaining the output ofsaid first current amplifier means at a constant potential; and controlmeans connected to said first inputs of each of said current amplifiermeans for controlling the output current amplitudes thereof.
 2. Theapparatus of claim 1 further comprising low-pass filter means forconnecting said control means to said first inputs of each of saidcurrent amplifier means.
 3. The apparatus of claim 1 wherein saidcontrol means comprises three signal sources whose output signals areadded to provide a control signal, one of said signal sources includingmeans for generating a reference signal of fixed amplitude, another ofsaid signal sources including means for generating a signal related tothe amplitude of the signals emitted by said current amplifier means,and the third of said signal sources including means for generating asignal related to the voltage difference across said terminals.
 4. Theapparatus of claim 3 further comprising low-pass filter means forconnecting said control means to said first inputs of each of saidcurrent amplifier means.
 5. The apparatus of claim 4 wherein said thirdsignal source comprises a difference amplifier means having inputsconnected across said terminals.
 6. The apparatus of claim 1 whereinsaid control means includes means for generating a signal whichrepresents the sum of the potential difference across said terminals andthe amplitude of the signals emitted by said current amplifier means.